Preheat your grill to medium-high, about 400 degrees F. Tear off four sheets of heavy-duty foil, each about 18 inches long. Lay them flat on your counter.

In a large bowl, combine the halved potatoes, sliced onion, olive oil, minced garlic, kosher salt, black pepper, and chopped rosemary. Toss until everything is evenly coated. The potatoes should look glossy but not swimming in oil.

Divide the potato mixture evenly among the four foil sheets, mounding it in the center. Dot each mound with butter pieces, then fold the foil over and crimp the edges tightly to seal. You want a good seal so steam stays inside, but leave a little room for air to circulate.

Place the packets directly on the grill grates, seam side up. Close the lid and cook for 20 minutes. The packets will puff up with steam - this is good.

Carefully flip each packet using tongs. Grill another 12 to 15 minutes until a knife slides easily into the largest potato piece. The bottom of the packet should have some golden, caramelized bits when you peek inside.

Transfer packets to a platter and let rest for 5 minutes. Open carefully - hot steam will escape. Sprinkle with fresh parsley and flaky salt, then serve directly in the foil or scoop onto plates.